This invention relates to a carbon fiber coated with a duplex film and to a method for the manufacture thereof.
Carbon fiber-reinforced plastics using high-strength, high-modulus carbon fibers as reinforcements are widely used in medical apparatuses, space and aeronautic vessels, land transportation machines, sports gear, etc. as structural materials having excellent mechanical properties such as high tensile strengths and Young's modulus. They, nevertheless, have a disadvantage to be deficient in thermal resistance because of the property of the plastics being used as matrix material. Then, to develope a composite material which excels in thermal resistance as well as in specific strength and specific elastic modulus studies are now under way on the use of metals or ceramics as a matrix for the composite material. Carbon fibers, however, when contact with metals or ceramics at high temperatures, tend to react with them to be heavily degraded in strength. Consequently, the composite material obtained can only acquire strength much lower than the expected level.
In order to prevent such reactions of carbon fiber with matrix materials as mentioned above or air, studies are also under way on coating ceramics such as silicon carbide and titanium carbide on carbon fibers. Such a coating is also supposed to improve the wetting ability of the carbon fiber surface to metal matrix.
In many cases such a coating tends to degrade the strength of carbon fibers by about 50% of before coating. This degradation seriously destroy the expected reinforcing effect of the carbon fiber. This degradation is considered to be due to the formation of faults such as grain boundaries playing a role as initiation points of rupture, and due to the tensile stress occurred and remained in the coatings after processing because of the difference in thermal expansion coefficient between the coating materials and the carbon fibers.
It also was found by the present inventors that when films of substantially sole SiC was coated on the carbon fibers, the strength of the coated fiber was reduced to 60% that of the original uncoated fiber. And it was suggested by them that a carbon fiber coated with a carbon riched SiC layer and SiC layer can possess a little higher strength than 60% that of the original fiber. After many investigations under various conditions, they have found that the selection of the appropriate composition and thickness is necessary for the production of much stronger carbide coated carbon fibers which maintain the strength before coating. They also have found that the structure and texture of the free carbon embedded in the film is essential for this strengthening effect of the carbon riched layer.